An ignition device is provided, including an ignition coil assembly with primary and secondary coils and an ignition plug connected to the secondary coil so as to, after the supply of a primary current to the primary coil, develop a high discharge voltage across the secondary coil by interruption of the primary current at a given ignition timing and thereby generate a discharge between electrodes of the ignition plug. The discharge voltage developed across the secondary coil and the magnitude of the discharge energy basically depend on the energization time of the primary coil.
Patent Document 1 discloses a technique to apply a superpose voltage to the ignition plug by another voltage booster during a discharge period after the ignition timing for the purpose of increasing the discharge period for reliable ignition. This technique enables, after the initiation of a discharge between the electrodes of the ignition plug with the application of the secondary voltage by the ignition coil assembly, a continuation of discharge current with the application of the superpose voltage to impart greater energy to air-fuel mixture.
For reduction of pumping loss and improvement of fuel efficiency, it is known practice to recirculate a relatively large amount of exhaust gas into a combustion chamber by external exhaust gas recirculation (abbreviated as “external EGR”) with the use of an exhaust gas recirculation passage from an exhaust system to an intake system or by internal exhaust gas recirculation (abbreviated as “internal EGR”) with the control of an overlap between an intake valve and an exhaust valve. However, such exhaust gas recirculation leads to a deterioration in the ignition performance of the ignition plug.